Technical challenges and process improvements moved older generation Thermo-Mechanically Controlled Processing (TMCP) pipes from coarse microstructures and presence of non-metallic inclusions and/or mid-thickness segregation, to finer, homogenized microstructures and improved properties. Despite such an improvement, local hard zones (LHZ) have recently been experienced in the Oil & Gas industry on large diameter line pipes manufactured from TMCP plates, resulting in an in-service failure due to sulfide stress cracking (SSC).

After thorough investigations, it was confirmed the root cause of the failure was ascribed to microstructure heterogeneities while manufacturing the TMCP plates. Sub-surface lower bainite was identified as crack initiation areas, leading to the failures.

This paper deals with the results of a research program initiated to evaluate the SSC resistance of the actual microstructure associated to these hard zones. The first step was to analyze an industrial pipe section that experienced SSC in service. The microstructure observed was successfully reproduced in laboratory on specimen blanks using Gleeble thermo-mechanical cycles. Then, specimens were tested in different conditions of the Region 3 of the pH-PH2S severity diagram of NACE MR0175 / ISO 15156 to evaluate their cracking susceptibility. As a result, the maximum hardness limit of 22 HRC (248 HV) historically specified in standards shall not be considered as a safe limit if local hard zones are present at the surface of the material. Thus, the threshold should be decreased to 210 HV for safe use in Region 3 sour environments.

Subject
Materials, Piping, Deformation, Microstructure, Crack initiation, Rolling, Mechanical failure, Steel, Sulfide stress cracking, Ferrite, Hardness, Heat treatment, Grain
Keywords:
Local Hard Zone (LHZ), TMCP, sour service, bainite, hardness, line pipe, SEM, EBSD
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2023
Association for Materials Protection and Performance (AMPP)
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